There some cities that get really boring at night and scary like Los Angeles, London, New York and more but Istanbul is not one of those cities. It is a very alive city and it doesn’t sleep. It is completely safe to walk alone at midnight and the people there are very friendly. They weren’t like that the past couple of years but they have changed.
One of the places that I like visiting at night is Sultan Ahmet. That place is beyond amazing with all the lights, the big fountain, the mosques and the darwish dance along with a good cup of tea/chay.
Ortakoy is one of the places that I like going to at night. Seeing the lights of the famous bridge sparkle in front of your eyes, eating their famous potato with all the fatty ingredients, just walking around or eating at one of their famous restaurants. It’s your choice but just walking around the place makes it fun.
Also Taksim/Istiklal street is a vivid and crowded streets filled with cafes, restaurants, shops and so many more but I don’t like going to it if I’m not staying in the area because it’s really annoying getting their, the streets are always jammed/crowded. Try it for a day and spend your time their from morning til evening.